The document discusses cells in the pancreas that produce insulin. It specifically focuses on beta cells, which are cells that express the Ptf1a gene. Ptf1a is essential for formation of the entire pancreas in both mice and humans, including beta cells that secrete insulin. By studying Ptf1a-expressing cells, scientists hope to better understand how to maintain or restore beta cell function. The document also discusses endoplasmic reticulum stress linking obesity to insulin resistance and type 2 diabetes. Studies in cell cultures and mouse models show that obesity causes ER stress, which leads to suppression of insulin signaling through JNK activation and IRS-1 phosphorylation.